Semana 1A Teoría endosimbiótica

Scientific concepts, discoveries, and nature phenomena

Endosymbiotic theory (core topic)

The origin of eukaryotic cell organelles—specifically mitochondria and chloroplasts—is explained by ancient symbioses. In this view, formerly free-living prokaryotes became incorporated into a larger host cell and evolved into organelles over time.

Origin of life (hypothesis presented)

According to Oparin and Haldane, life began with inorganic molecules in the primitive oceans, where reactions gradually produced increasingly complex organic molecules.

A proposed sequence is:

• Formation of first organic molecules, such as amino acids, nucleotides, carbohydrates, and fatty acids
• Assembly into macromolecules, such as:
  • Proteins from amino acids
  • Nucleic acids from nucleotides

The “primordial soup” idea suggests that the first cell arose when a nucleic acid (DNA-like) molecule became embedded within a lipid membrane, producing the simplest cell: an anaerobic prokaryotic cell.

Evolution driven by environmental change

As early cells multiplied:

• nutrients decreased
• Earth’s intense activity (e.g., electrical discharge, volcanic activity) promoted mutations in primitive DNA

Photosynthetic organisms emerged that:

• used carbon dioxide (CO₂) as a nutrient
• released oxygen (O₂) into the atmosphere

Later, aerobic organisms evolved that could use accumulated oxygen to produce energy.

Cell types described

Prokaryotic cells

• Examples: bacteria
• Contain circular DNA in a region of the cell
• Have no nuclear envelope (no nucleus)

Eukaryotic cells

• Found in animals and plants
• Have linear DNA surrounded by the nuclear envelope
• Include a nucleus, and the cytoplasm is compartmentalized into organelles
• Mentioned organelles include: Golgi complex, endoplasmic reticulum, ribosomes, etc.

Special emphasis:

• Mitochondria (in eukaryotes broadly)
• Chloroplasts (in eukaryotic plant cells)

Key functions of organelles

• Mitochondria: cellular respiration (breaks down glucose to obtain energy)
• Chloroplasts: photosynthesis (uses sunlight energy with CO₂ and water to obtain glucose)

Endosymbiotic theory mechanism (as described)

The described mechanism includes the following sequence:

• Roughly 2.5 billion years ago, oxygen increased due to photosynthetic organisms
• Some organisms evolved the ability to capture oxygen and use it for energy, and they proliferated
• A larger cell phagocytosed/ingested an aerobic organism, but the smaller organism was not digested and remained intact as a symbiont

Benefits:

• Host: improved energy production
• Symbiont: nutrients and protection

Over time:

• The aerobic symbiont is proposed to have become mitochondria
• If the incorporated prokaryote was photosynthetic, it is proposed to have become chloroplasts

Resulting cell types:

• Animal eukaryotes with mitochondria
• Plant eukaryotes with mitochondria + chloroplasts

Evidence cited for endosymbiosis

Mitochondria are described as sharing bacterial-like features, including:

• Independent circular DNA within mitochondria, similar to bacterial DNA
• Mitochondrial division by binary fission (splitting into two), like bacteria
• Ribosomes inside mitochondria similar to bacterial ribosomes
• Double membrane:
  • Inner membrane attributed to the incorporated prokaryote
  • Outer membrane attributed to the host cell

Researchers or sources featured

• Alexander Oparin
• John Haldane

(Also referenced generically as “scientists” and the “endosymbiotic theory,” but no other specific author names were given.)